One in three pregnancies is unplanned, the study found
Many women become pregnant because they do not use contraceptives properly, a study suggests.
Research carried out in France has found that one in three pregnancies is unplanned.
But of these, two out of three occur when couples are using contraceptives, such as the Pill or condoms.
The researchers said doctors should do more to ensure women who do not wish to become pregnant use the most suitable forms of contraception.
Dr Nathalie Bajos and colleagues at the Hospital de Bicetre, in Paris, based their findings on a survey of almost 3,000 women across France.
They found that 1,034 of these women had become pregnant unexpectedly. Half subsequently decided to have an abortion.
But of these, 65% said they had been using contraception: 21% said they had taken the Pill; 21% had used the "natural method"; 12% had used condoms; and 9% had used an intra-uterine device (IUD).
What this research shows most clearly is that there is often a mismatch between a women's contraceptive needs and the methods they use
The survey found that most of these women had become pregnant because they had not used the contraceptive properly.
For instance, 60% of those on the Pill said they became pregnant after forgetting to take a tablet.
One in three of those who used an IUD said it had been in the wrong position or had fallen out. Almost half of those who had used condoms said they had torn or slipped off.
But a large proportion of women said they did not know why their contraception had not worked.
This included one in five of those taking the Pill, over half of those using an IUD and one in five of those using condoms.
The doctors said the findings suggested many women were simply using the wrong type of contraception for them.
"What this research shows most clearly is that there is often a mismatch between a women's contraceptive needs and the methods they use," Dr Bajos said.
She pointed out that while the Pill is regarded as being very effective it is not suitable for every woman, such as those who do not have intercourse regularly or those whose lifestyle prevents them from taking tablets regularly.
Dr Bajos said doctors should work more closely with patients to ensure they are using the most suitable type of contraceptive.
The lessons of this study are equally applicable in the UK
"When a doctor prescribes a contraceptive, he or she should not think just of the theoretical perspective, but take into account the woman's life."
She added: "It is absolutely essential to differentiate between what are the theoretically most effective methods of contraception and what is the most practicable method for a particular women at a particular time in a particular relationship."
The fpa, formerly the Family Planning Association, welcomed the findings.
Its chief executive Anne Weyman said:
"The lessons of this study are equally applicable in the UK.
"To avoid unplanned pregnancies, it's essential women are aware of all the contraceptive choices available, and that professionals are fully trained to take into account the different factors that impact on effective contraceptive use, such as lifestyle, age and type of relationship."
The study is published in the journal Human Reproduction.